Metal Oxide Semiconductor (MOS) capacitors are needed in many analog integrated circuit applications requiring high capacitor density. A common approach to realizing a MOS capacitor is shown in FIG. 1. In FIG. 1, nMOSFET (n-Metal Oxide Semiconductor Field Effect Transistor) 102 has source 104 and drain 106 shorted to ground (substrate) 108 to form one plate of a capacitor, and gate 110 serves as the other plate.
In many applications, there is a need for a high density capacitor using a digital CMOS (Complementary Metal Oxide Semiconductor) process in which the voltage difference between the terminals of the capacitor is small. For example, FIG. 2 illustrates operational amplifier (OPAMP) 202, which is part of some larger circuit 222, such as, for example, an analog-to-digital converter, or a communication circuit such as an Ethernet PHY. OPAMP 202 comprises first differential stage 204 and a final output stage comprising nMOSFET 206 biased by current source 210, where the output signal is taken at output port 212 and input signals are applied at input ports 214 and 216. Miller compensation is applied to nMOSFET 206 by connecting capacitor 208 as shown in FIG. 2. Other stages, employing nMOSFETs, pMOSFETs, or both types of transistors, may be present in OPAMP 202, but for simplicity are not shown. The voltage difference between terminals 218 and 220 of capacitor 208 may be small, such as much less than 0.1 volts.
Operating capacitor 102 in its linear range usually requires a voltage difference across its terminals equal to or greater than its threshold voltage. For many process technologies, this threshold voltage is on the order of 0.7 volts. Even for process technologies where native MOS devices are available, the threshold voltage may still be about 0.1 to 0.2 volts. Consequently, using the structure of capacitor 102 in FIG. 1 for capacitor 208 in FIG. 2 may not be suitable.